A model to study the effects of partial coronary artery occlusion during atrial pacing in the pig has been worked out. In one technically satisfactory experiment during pacing with no and mild occlusion, the coronary sinus minus aortic concentration difference of total adenosine derivatives varied randomly between minus 5.6 and plus 6.1 micromoles/liter. With more severe constriction at a rate of 110 beats per minute coronary flow and arterial pressure dropped, R wave voltage on the ECG decreased, and a sample obtained during reactive hyperemia showed the coronary sinus concentration of adenosine derivates to be 17.7 micromoles/liter higher than arterial. Normal values in man have been established in twelve patients with lesions that do not involve the left ventricle, and the upper limits during pacing were found to be 1.2 micromoles/liter for the coronary sinus minus aorta difference of adenosine derivatives and 60 percent for the production of these compounds (coronary sinus minus aorta divided by aortic concentrations). In eight patients paced to angina, six have shown positive responses using the above criteria, the AV difference ranging between 1.0 and 10.5 micromoles/liter and the percent production ranging from 66 percent to 337 percent. Three of these patients showed lactate production, one showed decreased lactate extraction but no production, and two showed only a small decrease in lactate extraction.